Dissipation through spin Coulomb drag in electronic spin transport and optical excitations

Spin Coulomb drag (SCD) constitutes an intrinsic source of dissipation for spin currents in metals and semiconductors. We discuss the power loss due to SCD in potential spintronics devices and analyze in detail the associated damping of collective spin-density excitations. It is found that SCD contributes substantially to the linewidth of intersubband spin plasmons in semiconductor quantum wells, which suggests the possibility of a purely optical quantitative measurement of the SCD effect in a parabolic well through inelastic light scattering

Feasibility of approximating spatial and local entanglement in long-range interacting systems using the extended Hubbard model

We investigate the extended Hubbard model as an approximation to the local and spatial entanglement of a one-dimensional chain of nanostructures where the particles interact via a long range interaction represented by a `soft' Coulomb potential. In the process we design a protocol to calculate the particle-particle spatial entanglement for the Hubbard model and show that, in striking contrast with the loss of spatial degrees of freedom, the predictions are reasonably accurate. We also compare results for the local entanglement with previous results found using a contact interaction (PRA, 81 (2010) 052321) and show that while the extended Hubbard model recovers a better agreement with the entanglement of a long-range interacting system, there remain realistic parameter regions where it fails to predict the quantitative and qualitative behaviour of the entanglement in the nanostructure system.Comment: 6 pages, 5 figures and 1 table; added results with correlated hopping term; accepted by EP

Quantum test of the equivalence principle for atoms in superpositions of internal energy eigenstates

The Einstein Equivalence Principle (EEP) has a central role in the understanding of gravity and space-time. In its weak form, or Weak Equivalence Principle (WEP), it directly implies equivalence between inertial and gravitational mass. Verifying this principle in a regime where the relevant properties of the test body must be described by quantum theory has profound implications. Here we report on a novel WEP test for atoms. A Bragg atom interferometer in a gravity gradiometer configuration compares the free fall of rubidium atoms prepared in two hyperfine states and in their coherent superposition. The use of the superposition state allows testing genuine quantum aspects of EEP with no classical analogue, which have remained completely unexplored so far. In addition, we measure the Eotvos ratio of atoms in two hyperfine levels with relative uncertainty in the low $10^{-9}$, improving previous results by almost two orders of magnitude.Comment: Accepted for publication in Nature Communicatio

Studi di dettaglio della pericolosità sismica in aree ad elevata esposizione: un esempio per la Sicilia Orientale

La Sicilia Orientale è una delle aree che presenta i valori di pericolosità sismica tra i più elevati in Italia nella mappa di riferimento MPS04 (Gruppo di Lavoro MPS 2004). Considerando che questa è una zona con una elevata esposizione, sia in termini di Beni Culturali che di attività industriali, è stato compiuto uno studio particolareggiato che prendesse in considerazione i molti elementi conoscitivi resisi disponibili negli ultimi anni. E’ stato anche possibile utilizzare i notevoli progressi metodologici, resi possibili grazie allo sviluppo di codici di calcolo sempre più raffinati, che, insieme alle moltiplicate capacità di calcolo dei processori oggi disponibili, rendono queste valutazioni realizzabili in tempi molto brevi e quindi consentono anche di sviluppare modelli con un grado di complessità elevato. I calcoli sono stati realizzati utilizzando la versione 2007 del software CRISIS, un codice Open Source e liberamente distribuito, messo a punto presso l’Università Autonoma del Messico (Ordaz et al., 1999), in particolare usando l’applicazione web sviluppata nell’ambito del progetto INGV-DPC S2 (http://nuovoprogettoesse2.stru.polimi.it) che aggiunge alla versione desktop alcune importanti funzionalità, quali la possibilità di condividere dati tra diversi utenti, l’indipendenza dal sistema operativo utilizzato per lo sviluppo dell’applicazione, l’esecuzione dei calcoli da parte del server liberando le risorse locali dell’utente. CRISIS si basa essenzialmente sull’approccio standard di Cornell (1968) alla stima probabilistica della pericolosità sismica e consente di utilizzare due diversi modelli di sismicità: quello “poissoniano” (vale a dire di eventi indipendenti tra loro e con modalità di rilascio “costanti” nel tempo) e il modello “caratteristico” (che si applica a faglie sismogenetiche per le quali si ipotizza il rilascio di energia attraverso terremoti di magnitudo prefissata e con periodo di ritorno noto). Secondo la prassi standard e consolidata a livello internazionale per la stima della pericolosità sismica (SSHAC, 1997), è stato seguito un approccio cosiddetto ad albero logico per considerare tutte le possibili alternative nella scelta dei modelli utilizzati nel calcolo e valutare l’incertezza epistemica delle stime prodotte derivante dall’uso di queste opzioni

Long-term observations of the pulsars in 47 Tucanae - II. Proper motions, accelerations and jerks

This paper is the second in a series where we report the results of the long-term timing of the millisecond pulsars (MSPs) in 47 Tucanae with the Parkes 64-m radio telescope. We obtain improved timing parameters that provide additional information for studies of the cluster dynamics: a) the pulsar proper motions yield an estimate of the proper motion of the cluster as a whole ($\mu_{\alpha}\, = \, 5.00\, \pm \, 0.14\, \rm mas \, yr^{-1}$, $\mu_{\delta}\, = \, -2.84\, \pm \, 0.12\, \rm mas \, yr^{-1}$) and the motion of the pulsars relative to each other. b) We measure the second spin-period derivatives caused by the change of the pulsar line-of-sight accelerations; 47 Tuc H, U and possibly J are being affected by nearby objects. c) For ten binary systems we now measure changes in the orbital period caused by their acceleration in the gravitational field of the cluster. From all these measurements, we derive a cluster distance no smaller than $\sim\,$4.69 kpc and show that the characteristics of these MSPs are very similar to their counterparts in the Galactic disk. We find no evidence in favour of an intermediate mass black hole at the centre of the cluster. Finally, we describe the orbital behaviour of the four "black widow" systems. Two of them, 47 Tuc J and O, exhibit orbital variability similar to that observed in other such systems, while for 47 Tuc I and R the orbits seem to be remarkably stable. It appears, therefore, that not all "black widows" have unpredictable orbital behaviour.Comment: 21 pages in journal format, 9 figures, 4 tables, accepted for publication in MNRAS, several clarifications made and typos fixe

Probabilistic seismic hazard assessment in the high-risk area of south-eastern Sicily (Italy)

A probabilistic seismic hazard analysis (PSHA) was carried out for the SE sector of Sicily, an area characterized by highest levels of seismic hazard in Italy and high exposure, both in terms of cultural heritage and of critical industrial facilities. Compared to the Italian reference PSH map (MPS04), this study is based on most updated information about regional seismic sources and ground-motion attenuation. Epistemic uncertainties associated with the input elements of the computational model were taken into account following a logic-tree approach. Special care was devoted to define the regional source zones model by considering four alternative models, which share the zones defining the boundary conditions of the study area but differ in the seismotectonic characterization of SE Sicily. Seismic hazard was assessed in terms of PGA, PGV, acceleration and displacement elastic response spectra on rock for four return periods (30, 50, 475, 975 years). A disaggregation analysis was then performed for some sites of interest. Results confirm the very high hazard of the area, with expected values of PGA (at 10% probability of exceedance in 50 years) slightly higher than the reference MPS04 map. Strong differences emerge instead between the acceleration response spectra of this study and the reference ones, for the longest return periods